1. Field of the Invention
The present invention relates to an apparatus and method for delivering a breathing gas to a user at alternating levels of pressure as a treatment for respiratory conditions such as sleep apnea.
2. Discussion of the Background Art
The sleep apnea syndrome affects some 1% to 5% of the general population and is due to upper airway obstruction during sleep. The direct consequences of sleep apnea are sleep fragmentation, partial cessation of ventilation and oxyhemoglobin desaturation. These in turn translate into daytime somnolence, cardiac arrhythmia, congestive heart failure and a variety of other health as well as cognitive dysfunctions. All of these have secondary social and behavioral effects which can result in increased patient morbidity as well as possible mortality if they are engaged in activities which require alertness (such as driving a car).
The causes of upper airway obstruction are varied but may include anatomical changes leading to a narrowing of the pathway, loss of muscle tone and/or increased weight of the structures. Age and obesity appear to be risk factors suggesting that an excess of soft tissue in the neck may provide sufficient pressure on internal structures to compromise the patency of the airway.
Treatment has involved a variety of surgical interventions including uvulopalatopharyngoplasty, gastric surgery for obesity, maxillo-facial reconstruction or even tracheostomy. All of these procedures have the risk of significant morbidity. A more benign treatment but one which requires some behavioral adjustment is that of nasal continuous positive airway pressure (nCPAP or just CPAP). In its simplest form, this treatment involves applying positive pressure to the airway using an airflow generator to force the passage to remain open. If used consistently during sleep, symptoms of sleep apnea can be successfully mitigated.
Some patients, however, are nonresponsive or noncompliant with CPAP treatment due to its continuous nature. This is especially true if the CPAP prescription pressure is relatively high. For these individuals a bilevel therapy is a more reasonable alternative. Pressure cycles from a high level during inhalation (IPAP) to a low level (EPAP) to facilitate exhalation while at the same time continuing to provide some nominal pressure support. This is also useful for individuals who have some form of compromised respiration such as a weakness of the diaphragm muscle due to disease or spinal injury where continuous pressure may be problematic.
In accomplishing the IPAP to EPAP switch several techniques have been used by the prior art. Some examples include the use of weighted bellows, pressure-reducing valves coupled to pneumatic sources, constant speed blowers coupled with valving, injector drives, linear driven pistons, nonlinearly driven pistons, and spring loaded bellows. Most systems have used constant speed blowers coupled with valving to effect rapid pressure changes requiring minimal energy. Typically the blower will rotate to a speed sufficient to provide the higher IPAP pressure and adjust the pressure downwards using a solenoid controlled exhaust valve. The valve position is changed by a small amount to compensate for increased flow demand and by a large amount to shunt air away from the patient and drop the pressure to EPAP levels.
Variable speed blowers have not typically been used for bilevel flow generators heretofore for several reasons. First, the mass of the blower-motor assembly prevents its rapid acceleration and deceleration which in turn causes suboptimal performance with respect to the work of breathing (i.e., the inhalation or exhalation of a given patient is not supported in real time). Second, off-the-shelf motor controllers are typically not suitable for rapid blower performance and typically require external sensing devices (e.g., hall effect sensors) adding to cost. Third, variable speed motors are traditionally direct current (DC) motors which dissipate large amounts of heat thereby requiring a means of heat dissipation and making miniaturization of the device more difficult.
The current invention makes it possible to employ a variable speed blower in a bilevel flow generator through the use of a microprocessor-controlled alternating current (AC) synchronous permanent magnet motor coupled to a low inertia centrifugal impeller and powered by a low wattage constant voltage switching power supply. The device will rotate the impeller at the rotational frequency of its field which may be changed in accordance with the interrupt rate of a timer circuit in the microprocessor. A method of changing rapidly from one frequency to another without using sensors while maintaining proper operation of the system is also the subject of this disclosure. A method whereby field current may be related to system variables (i.e. flow) will also be disclosed.
A first aspect of the present invention is generally characterized in an apparatus for delivering a breathing gas to a user including a blower having an alternating current (AC) motor and an impeller rotated by the AC motor to deliver a breathing gas to a user, and a blower control system providing a control signal of variable frequency and amplitude to the AC motor. The frequency and amplitude of the control signal are adjusted periodically by the blower control system to cause operation of the AC motor to alternate between an inhalation mode wherein the impeller is rotated at a first speed generating an inhalation positive airway pressure and an exhalation mode wherein the impeller is rotated at a second speed generating an exhalation positive airway pressure. The blower control system preferably includes a microcontroller which generates a plurality of control signals in the form of pulse width modulated signals having sinusoidally weighted duty cycles. The microcontroller preferably also includes a compare unit with a timer and a plurality of compare registers so that the sinusoidally weighted pulse width modulated signal can be created based on a comparison of a timer value with a compare register value retrieved from a memory device. The compare timer preferably has a variable interrupt rate defining a frequency of the pulse width modulated signal. The amplitude and frequency of the pulse width modulated signals are preferably adjusted by the microcontroller using the Bresenham algorithm.
A second aspect of the present invention is generally characterized in a method of generating bilevel positive airway pressure for respiratory therapy using a gas flow generator having a blower powered by an alternating current (AC) motor. The method includes the steps of applying a motor control signal having an amplitude and a frequency to the motor to produce a blower speed, determining whether a speed change is required for the blower, determining a target amplitude and a target frequency corresponding to a target speed, and adjusting the amplitude and frequency of the motor control signal in steps until the amplitude and frequency correspond with the target amplitude and the target frequency. One of the amplitude and frequency is incremented at a first step interval and the other is incremented at a second step interval which is a multiple of the first step interval, for example using the Bresenham algorithm. Preferably, the method also includes the steps of adding excess amplitude before adjusting the motor control signal and removing the excess amplitude after the target frequency has been achieved so as to maintain synchronization.
In a preferred embodiment, the invention utilizes a low inertia permanent magnet synchronous three-phase AC motor that is supplied by pulse width modulated (PWM) pulses that resemble a sine wave of varying frequency and amplitude. The process of acceleration and deceleration involves moving from frequency A, amplitude A to frequency B, amplitude B in an optimal linear fashion, preferably using the so-called Bresenham algorithm. This is preferably coupled with a tuned increase of the amplitude during the acceleration process that will produce the acceleration using minimum current. During deceleration the process can be accomplished in reverse fashion using a tuned decrease of the amplitude coupled with a special shunt circuit to prevent power supply voltage changes. These changes in amplitude overlay a current feedback mechanism used to prevent loss of synchronization of the motor by changing amplitude. Speed changes are also preferably timed so as to prevent desynchronizations.
The above and other features and advantages of the present invention will be further understood from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings in which like numerals denote like parts.